1. Field of the Invention
The present invention relates to a semiconductor device and a process for producing the same.
2. Description of the Prior Art
Conventionally, in order to shorten a minority carrier lifetime in the manufacture of silicon diodes, transistors, and thyristors which require high-speed switching characteristics, either an impurity such as Au is diffused in a semiconductor substrate or an electron beam is irradiated on the semiconductor substrate. The electron beam irradiation technique is frequently used because the process is easy to control. When the semiconductor substrate is irradiated with an electron beam, crystal defects occur therein. The crystal defects serve as recombination sites for capturing the minority carriers, so that the minority carrier lifetime is shortened. The change in the lifetime of the minority carriers due to electron beam irradiation is given by: EQU 1/.DELTA..tau.=1/.tau.-1/.tau..sub.0 =R.sigma.V.sub.th .phi.=K.phi. Ne=R.phi..multidot.(1/.tau.)=.sigma.V.sub.th Ne . . . (1)
where
.tau..sub.0 : the minority carrier lifetime before electron beam irradiation PA1 .tau.: the minority carrier lifetime after electron beam irradiation PA1 R: the introduction ratio of the recombination centers introduced by electron beam irradiation PA1 .sigma.: the capture cross section of the recombination center PA1 V.sub.th : the thermal carrier velocity PA1 K: the damage coefficient due to use of the electron beam PA1 Ne: the recombination center density (cm.sup.-2) PA1 .phi.: the electron beam irradiation dose (cm.sup.-2)
As is apparent from the above equation, the term 1/.DELTA..tau. can be freely controlled by changing the electron beam irradiation dose .phi..
However, electron beam irradiation can only be conventionally performed in units of wafers or chips (semiconductor devices). Therefore, the minority carrier lifetime cannot be selectively controlled within a single semiconductor device.